German version of PORPUS-P: Reference values, reliability, construct validity, discriminant and concurrent validity

Background and Objectives The PORPUS-P is a short questionnaire for measuring prostate-specific related quality of life. The aim of the present study was to test psychometric properties (reliability, concurrent validity) of the German version of this instrument when applied to the general population. Furthermore, discriminant validity was tested by application of this questionnaire to patients with erectile dysfunction, prostate cancer patients with radiotherapy or radical prostatectomy. Patients and Methods Data sets of three studies were combined (San BKK-study, ProCaSP-study, CHEDY-study). In total, the study sample consisted of 988 men from the general population, 87 prostate cancer patients with radiotherapy, 274 prostate cancer patients with radical prostatectomy, and 48 men with erectile dysfunction. All men were assessed with PORPUS-P, EQ-5D, and EORTC QLQ-C30. Psychometric properties were tested with between-subgroup-differences (Mann-Whitney U-test, Kruskal-Wallis H-test, binary logistic regression analysis), computing age adjusted means, cronbach's alpha, correlation coefficients, and estimating floor and ceiling effects. Results The study sample consists of 988 men from the general population (mean age: 56 yrs), 87 prostate cancer patients with radiotherapy (67 yrs), 274 prostate cancer patients with radical prostatectomy (63 yrs), and 48 men with erectile dysfunction (58 yrs). Our analyses indicate that PORPUS-P is a valid tool to assess prostate-specific quality of life: Full-scale correlations between the instruments (PORPUS-P vs. EORTC QLQ-C30 global health, EQ-5D VAS and EQ-5D index value; rs range= 0.518-0.630; p<0.001) indicate validity as do correlations of key subscales. PORPUS-P is able to discriminate between different health status, age groups and categories of sociodemographic variables. Floor effects were seen regarding the answering scheme of some of the PORPUSitems. Conclusion Our data give evidence of the construct, discriminant, and concurrent validity as well as internal consistency. However, some PORPUS-items showed floor effects. In general, floor effects were more strongly pronounced in reference men than in erectile dysfunction or prostate cancer patients. We recommend the use of PORPUS-P in cases of study situations in combination with other generic questionnaires such as EORTC QLQC30. In cases of clinical routine PORPUS-P can be used as a "stand-alone solution".